The present invention relates generally to automatic transmissions for motor vehicles, and, more particularly, to the control and management of power-off or coasting downshifts in such transmissions.
It is difficult to calibrate excellent shift quality under all circumstances for coasting or power-off downshifts in synchronous automatic transmissions. Such calibrations are especially difficult under low-speed conditions where vehicle noise levels are low and gear ratio steps are largest. Coasting downshifts present the largest inhibitor to a fully synchronous automatic transmission design. Control strategies require calibration flexibility to ensure consistent, smooth downshifts under all operating conditions, but conventional gear shift control strategies are insufficient to meet the current requirements for coasting shift quality and cost.
In an automatic transmission a hydraulic pump, driveably connected to the engine shaft, produces fluid flow in a hydraulic system, which controls and actuates the ongoing and off-coming clutches, whose coordinated operation produce downshifts during coasting conditions. The speed of the pump, which has a constant displacement, can vary between about 500-7500 rpm; therefore the flow rate produced by the pump varies over a wide range. But at low speed during coast down gear shifting, the pump's operating efficiency is low and the flow rate it produces may be insufficient compared to that required to control the gear shifts. A conventional prior art solution is to control the engine during coast down shifts such that it operates at a higher speed than would otherwise be required but for the fluid flow rate requirement. Unfortunately this strategy reduces fuel economy.
Furthermore, if the speed of the torque converter turbine is greater than engine speed during coast down shifts, the resulting gear shifts are unacceptably harsh and readily noticed by vehicle occupants because engine speed is low.